For example, JP H.11-249452 A describes this kind of image forming apparatus. This image forming apparatus includes a plurality of developing units of respective colors aligned in parallel with each other. When forming a multi-color image, developing operations are sequentially performed by the plurality of developing units.
In this apparatus, after new toner is supplied or after a toner cartridge being used is replaced with a new toner cartridge, an image quality may be deteriorated as using time elapses (as the number of image formation sheets is increased). The deterioration of an image quality over time is mainly caused due to deterioration of toner in a black developing unit which is most frequently used, introduction of foreign substances such as paper dusts into a developing unit in which the developing operation is first performed, and the like.